The present invention relates to a method of producing a cast steel article by using a permeable mold such as a sand mold, more particularly to a method of producing a thin cast steel article made of stainless steel, heat-resistant steel, etc. without suffering from casting defects such as insufficient filling, shrinkage cavities, blow holes, metal penetration, etc., and an apparatus for such a method.
Since exhaust equipment members for automobiles, for instance, prechambers, port liners, exhaust manifolds, turbocharger housings, exhaust outlets connected right under turbochargers, and parts for exhaust gas-cleaning members such as exhaust gas-cleaning catalytic converters, etc. are produced by metals having high heat resistance and oxidation resistance. For such applications, stainless steel and heat-resistant cast steel have recently been attracting much attention and have started to be put into practical use. However, since these stainless steel and heat-resistant cast steel generally have high melting points, their melts poured into a sand mold are easily solidified upon coming into contact with the cavity wall of the sand mold. Also, even before solidification, their melts show high viscosity (poor fluidity), resulting in insufficient filling of the mold cavity.
Also, in a case where thin cast articles having complicated shapes are produced, the air and gas generated from the sand mold are likely to be introduced into the resulting cast articles as blow holes, gas defects, etc. Further, since their melts are poured at high temperatures, they are easily reacted with the sand mold, resulting in metal penetration, which leads to provide the cast products with rough surfaces. Accordingly, it has been extremely difficult to produce cast articles having portions as thin as 5 mm or less from stainless steel and heat-resistant cast steel without defects.
One method for overcoming the above problems is a so-called lost wax method utilizing a mold made of ceramics. In this method, the mold is heated to 700.degree.-900.degree. C. before pouring the melt to reduce the cooling speed of the melt in the mold, thereby preventing the flowability of the melt from decreasing. However, this method is costly because expensive ceramic molds are used.
An alternative method for improving the flowability of the melt is a vacuum casting method in which casting is conducted under reduced pressure in a mold cavity. For instance, Japanese Patent Publication No. 60-35227 discloses such a vacuum casting method called "CLAS method," which is recently utilized for producing thin castings. This method is attracting much attention as a method for producing thin cast articles. However, in this conventional vacuum casting method, the mold is immersed in a melt. Accordingly, complicated structure and mechanism are necessary for holding and immersing the mold in the melt.
Japanese Patent Laid-Open No. 61-180642 discloses a vacuum control means in which a melt is poured into a permeable mold after a chamber containing the permeable mold is sucked. However, since the entire portion of the permeable mold is sucked, the disturbance of flow is likely to take place in the melt, resulting in cast steel article with blow holes, etc. Also, the chamber may be exploded due to the gas generated from the mold.
Japanese Patent Laid-Open No. 57-31463 discloses the production of thin castings in which a melt is poured into a cavity while evacuating through a hole positioned most distant from a sprue to increase the flowability of the melt. However, since suction is conducted through all of the inner surface of the cavity in this process, large suction effect cannot be achieved in necessary portions, and the melt flow is likely to be disturbed, resulting in the inclusion of air, slag, etc. into the cast articles.
Japanese Patent Laid-Open No. 60-56439 discloses a gypsum mold for vacuum-casting thin articles free from casting defects due to insufficient filling and gas defects due to the inclusion of the air, which is provided with a filter made of a refractory material having better gas permeability than that of the gypsum in an area ranging from a last-filled portion of the cavity to an outer surface of the gypsum mold, thereby increasing the pressure reduction effect in the cavity. Since gypsum is used for the mold, the penetration of the melt into the mold hardly occurs. However, a lot of steps are needed to produce the gypsum mold, resulting in poor productivity as compared with the sand mold. Also, since the gypsum mold does not have a good gas permeability, back pressure in the cavity increases in the process of casting, resulting in poor flow of the melt.
Japanese Patent Laid-Open No. 4-147760 discloses a sand mold for producing thin castings free from gas defects by a vacuum casting method, which is provided with a suction guide defining a suction path between a portion of a cavity from which the suction should be conducted and the outside, thereby locally evacuating the above cavity portion to increase the suction effect and also to suck the gas generated from the sand mold during the casting process. However, since the suction guide is embedded in the sand mold, the production of the sand mold is complicated. In addition, since the suction path defined by the suction guide is not much better in permeability than the sand mold, large suction effects cannot be obtained.